The expression “digital position transmitter” used in this disclosure relates to a system which controls one or more output signals corresponding to a plurality of input signals. Some of the input signals represent a static or dynamic set state while others of the input signals characterize a static or dynamic actual state. The output signals are used to make the actual state match the set state. The algorithm for this purpose is implemented in software in a microcontroller. In general, the output signals—with or without the use of auxiliary power—control the position of an actuating element.
Disturbance variables are superimposed on the input signals. These include the noise of the input signals as well as hysteresis, adhesion resistances and sliding resistances in the actuating elements. These disturbance variables—particularly when the desired position accuracies are high—result in undesirable oscillations of the positioner. These are counteracted by a dead band which suppresses changes in the output signals of the position transmitter as soon as the absolute value of the static or dynamic differences between the set state and the actual state falls below specific limit values. The dead band applies not only to changes in the set value but also to changes in the actual state.
The digital position transmitter comprises a digital regulator which is equipped with a dead band which symmetrically includes the set value. When a digital position transmitter such as this is included in a control loop, it appears for the superordinate control system to be defective when there is no change in the actuating element and thus in the actual state in response to a change, in particular a small change, in the set state. For the control system, a fault is present since the actuating element “does not react”. Depending on the safety requirement, this false alarm can lead to the process installation being brought to a safe state, in any case unnecessarily disturbing the technical process.